1. Field of the Invention
This invention concerns an electrochromic display device (hereinafter simply referred to as ECD) and, more specifically, it relates to an improvement in the counter electrode material therefor.
2. Description of the Prior Art
Recently, ECDs have been noted as the display device since they are free from view angle-dependency, capable of large area display and provide memorizable display. As the display material for ECD, inorganic material such as tungsten oxide and Prussian blue, as well as organic material such as heptylviologen dibromide have been used. The feature of ECD, in view of the operation principle, resides in the coloring/discoloring reactions of electrochromic substance (hereinafter simply referred to as EC substance) due to electrochemical reactions. In this case, it has to be noted that a reaction reverse to that on the display electrode occurs in the counter electrode. Accordingly, it is required for the counter electrode that it can stably perform effectively reversible oxidation-reduction reaction, in which less fluctuation occurs in the potential of the counter electrode itself accompanying the reaction and it rapidly tends to converge to a predetermined potential, etc.
As one of the means for satisfying such conditions, the same EC substance is formed by the same amount as in the display electrode to the counter electrode. In this case, since the reactions occurring at the two electrodes are quite opposite in the direction and equal with each other in the reaction amount, the device can be driven with ease. However, such a symmetrical type ECD has a drawback in that, if the reaction amount differs between the display electrode and the counter electrode due to external factors such as temperature change and light irradiation or internal factors such as defoliation of the display electrode, colored or discolored states are gradually accumulated along with the repeating cycles of operation, till the device can not operate at last.
As another means, a substance exhibiting stable oxidation-reduction reaction such as Prussian blue and a substance of an extremely large specific surface area such as carbon black are mixed together with a resin or like other binder and coated on the counter electrode. In this case, although a preferred ECD can be obtained, it involves a problem in view of the cell structure as in the symmetrical type ECD described above. Namely, a white background plate has to be disposed in an electrolyte between the display electrode and the counter electrode, so that the counter electrode may be consealed from the display surface. A ceramic plate or a fluoro resin molded plate of 0.2 to 0.5 mm thick has often been used as the white background plate. However, the white background plate occupies a most part of the entire cost of the ECD cell and also results in a problem that the thickness of the ECD cell can not be decreased to less than the thickness of the white background plate.